Actuator for operating a friction fastening element

Abstract

To achieve both an improvement in responsiveness at the fastening of a friction fastening element and fine controllability of a fastening force in an operation actuator of a friction fastening element.SOLUTION: An operation actuator 2 of a friction fastening element 1 has: an electric motor 21; a screw mechanism 22 for converting a rotation force of the electric motor 21 to a linear motion; a piston 23 for pressing a friction plate 13; and an oil charging chamber 26 in an oil-tight state charged with a working fluid for transmitting the linear motion of the screw mechanism 22 to the piston 23. The electric motor 21 is subjected to a stroke control of controlling a stroke amount of the piston 23 by controlling a rotation number of the electric motor 21 when moving the piston 23 to a zero-clearance position a2 from a prescribed release position a1, and also subjected to a torque control of controlling the transmission torque of the friction fastening element 1 by controlling output torque of the electric motor 21 when moving the piston 23 to a prescribed fastening position a3 from the zero-clearance position a2.SELECTED DRAWING: Figure 5

Description

Technical Field

[0001] The present invention relates to an operating actuator for a friction engagement element mounted on a vehicle such as an automobile.

Background Art

[0002] A friction engagement element mounted on a vehicle is mounted, for example, on a transmission connected to a drive source such as an engine and including a plurality of planetary gear sets. The transmission is configured to switch a power transmission path passing through each planetary gear set by selectively engaging a plurality of friction engagement elements to achieve a plurality of forward gears and usually one reverse gear.

[0003] As the friction engagement element, there is known a friction engagement element including a plurality of friction plates and a piston that presses the plurality of friction plates to engage the friction engagement element when hydraulic pressure is supplied to a hydraulic chamber, and that uses a hydraulically actuated piston to engage and disengage the friction engagement element.

[0004] When this type of friction engagement element is engaged, the clearance between the plurality of friction plates becomes zero, and the position of the piston is set to a zero clearance position where the tip is in contact with or substantially in contact with the friction plate (hereinafter also referred to as the "zero clearance state") without the piston pressing the friction plate, so that the stroke from the release position to the engagement position is pre-compressed, which may improve the responsiveness during engagement.

[0005] When the friction engagement element is engaged, in the release-side stroke region from the release position of the piston to the zero-clearance position, it is desirable to increase the stroke speed of the piston due to the requirement for responsiveness. In the case of a hydraulically actuated piston, it is conceivable to supply a high pressure to the hydraulic chamber. The hydraulic pressure supplied to the hydraulic chamber is generally generated by an oil pump driven by an engine and controlled by a hydraulic control device equipped with a plurality of solenoid valves. When a high pressure is supplied to the hydraulic chamber, a high hydraulic pressure acts on the piston even after the piston reaches the zero-clearance position, and there is a risk of shock due to the sudden engagement of the friction engagement element.

[0006] Patent Document 1 discloses, as a friction engagement element, instead of a hydraulic control device such as an oil pump and a plurality of solenoid valves, an electric motor, a conversion mechanism that converts the rotational motion of the electric motor into a linear motion, and a hydraulic pressure generating piston different from the piston that is fitted into an oil-filled chamber in an oil-tight state filled with hydraulic oil that transmits the linear motion to the piston. In order to suppress the shock caused by the sudden engagement of the friction engagement element, it is conceivable to increase only the stroke speed of the piston in a preset release-side stroke region by rotational speed control that controls the rotational speed (hereinafter also referred to as "rotational speed") of the electric motor of the operating actuator.

Prior Art Documents

Patent Documents

[0007]

Patent Document 1

Summary of the Invention

Problems to be Solved by the Invention

[0008] In order to perform good fastening control of the friction fastening element, it is necessary to precisely control the transmission torque of the friction fastening element while the piston moves in the fastening side stroke region from the zero clearance position to the full fastening position where the friction fastening element is fully fastened.

[0009] However, in the fastening side stroke region compared to the release side stroke region, the ratio of the change amount of the torque of the friction fastening element to the stroke amount is large, and the controllability is poor. Therefore, as described above, when the fastening control of the friction fastening element is executed by controlling the stroke amount of the piston by controlling the rotational speed of the electric motor, precise fastening control is not executed in the fastening side stroke region, and there is a risk of shock due to an unexpected change in the transmission torque.

[0010] The present invention provides an operating actuator for a friction fastening element that can achieve both an improvement in responsiveness during fastening of the friction fastening element and precise controllability of the fastening force.

Means for Solving the Problems

[0015] The present invention An operating actuator of a friction fastening element that is provided between a drum and a hub and presses a plurality of friction plates that alternately engage with the drum and the hub to fasten them to each other, comprising an electric motor, a screw mechanism that converts the rotational force of the electric motor into linear motion, a piston that presses the friction plates, and an oil-filled chamber in an oil-tight state filled with hydraulic oil that transmits the linear motion of the screw mechanism to the piston, wherein the screw mechanism includes a screw shaft and a nut that moves axially on the screw shaft and transmits a pressing force to the piston via the oil-filled chamber, the electric motor, when moving the piston from a predetermined release position to a zero-clearance position where the clearance between the friction plates is filled and the plurality of friction plates are in a zero-clearance state, stroke control is executed to control the stroke amount of the piston by controlling the rotational speed of the electric motor, and when moving the piston from the zero-clearance position to a predetermined fastening position, torque control is executed to control the transmission torque of the friction fastening element by controlling the output torque of the electric motor, The screw mechanism is constituted by a ball screw mechanism, The screw shaft includes a first pitch region in which a first pitch is formed on one side in the axial direction and a second pitch region in which a second pitch is formed on the other side in the axial direction continuously with the first pitch region, and the second pitch is formed to be smaller than the first pitch, The first pitch and the second pitch are set so as to switch at the zero clearance position to provide an operating actuator of a friction fastening element.

[0016] According to the present invention, in the release-side stroke region where the ratio of the stroke change amount to the change amount of the transmission torque of the friction fastening element is larger than that in the fastening-side stroke region, by performing stroke control of the electric motor, the responsiveness during fastening of the friction fastening element can be improved. Also, in the fastening-side stroke region where the ratio of the change amount of the transmission torque of the friction fastening element to the stroke change amount is larger than that in the release-side stroke region, by performing torque control of the electric motor, precise control of the transmission torque during fastening of the friction fastening element can be executed. Since the transmission torque of the friction fastening element is proportional to the hydraulic oil pressure, the hydraulic oil pressure is proportional to the nut pressing force, and the nut pressing force is proportional to the motor torque, for example, based on the transmission torque, hydraulic oil pressure, and nut pressing force of the friction fastening element, by performing torque control of the electric motor, the transmission torque of the friction fastening element can be directly controlled compared to the case of stroke control. Furthermore, since the operating actuator is composed of a ball screw mechanism, an oil-filled chamber in an oil-tight state, and a piston, it is possible to suppress energy loss for pressure holding in the unfastened state, unlike the case of driving a hydraulic pump to suppress a decrease in hydraulic pressure due to oil leakage from a solenoid valve or the like for hydraulic pressure holding in the unfastened state. Also, By using a so-called variable pitch ball screw mechanism in the screw mechanism, which has a first pitch and a second pitch with different pitches, it is possible to increase the stroke speed of the piston in the release side stroke region and reduce the stroke speed while ensuring the pressing force required for fastening the friction fastening element in the fastening side stroke region without increasing the size of the motor.

[0017] If the output of the electric motor is kept constant and the pitch of the screw shaft is set so as to obtain the maximum required rotational speed, the maximum required torque cannot be obtained, and it may be considered to increase the output of the electric motor in order to obtain the maximum required rotational speed and the maximum required torque. However, an increase in the output of the electric motor leads to an increase in the size of the electric motor. On the other hand, for example, by setting the first pitch to be the maximum stroke speed at the time of fastening and setting the second pitch to be the maximum required torque at the time of fastening, a smaller motor can be used compared to the case of mounting a large motor capable of outputting the maximum rotational speed and the maximum required torque. As a result, it is possible to use a small-sized motor with good mountability while improving the stroke speed of the piston in the release side stroke region and ensuring the fastening torque required in the fastening side stroke region.

[0018] Compared with the case of using a sliding screw mechanism in the screw mechanism, the contact area can be reduced and the screw efficiency can be improved. As a result, the motor torque is reduced and the electric motor can be miniaturized.

[0019] It further includes zero clearance position detection means for detecting the zero clearance position. The zero clearance position detection means may be configured to detect the zero clearance position of the piston at the rise of the hydraulic pressure in the oil filling chamber.

[0020] According to this configuration, since the zero-clearance position detecting means detects the position of the piston when the hydraulic pressure in the oil filling chamber rises as the actual zero-clearance position, for example, even when there is a deviation between the actual release-side stroke region and the release-side stroke region at the time of design due to the temperature characteristics of the friction plate, manufacturing errors, wear, etc., the actual zero-clearance position can be accurately detected.

[0021] The zero-clearance position detecting means may be constituted by a hydraulic pressure switch.

[0022] According to this configuration, the zero-clearance position of the piston can be detected with a less expensive configuration compared to the case of using a hydraulic pressure sensor or the like.

[0023] The zero-clearance position detecting means may be constituted by a hydraulic pressure sensor.

[0024] According to this configuration, more precise controllability can be obtained compared to the case of using a hydraulic pressure switch.

[0025] Another aspect of the present invention is an operating actuator of a friction fastening element that is provided between a drum and a hub and presses a plurality of friction plates that alternately engage with the drum and the hub to fasten them to each other, comprising an electric motor, a screw mechanism that converts the rotational force of the electric motor into linear motion, a piston that presses the friction plates, and an oil-filled chamber in an oil-tight state filled with hydraulic oil that transmits the linear motion of the screw mechanism to the piston, wherein the screw mechanism has a screw shaft and a nut that moves axially on the screw shaft and transmits a pressing force to the piston via the oil-filled chamber, the electric motor executes stroke control for controlling the stroke amount of the piston by controlling the rotational speed of the electric motor when moving the piston from a predetermined release position to a zero-clearance position where the clearance between the friction plates is filled and the plurality of friction plates are in a zero-clearance state, and executes torque control for controlling the transmission torque of the friction fastening element by controlling the output torque of the electric motor when moving the piston from the zero-clearance position to a predetermined fastening position, It further includes zero-clearance position detecting means for detecting the zero-clearance position, The zero-clearance position detecting means detects the zero-clearance position of the piston when the ratio of the change amount of the transmission torque of the friction fastening element to the change amount of the stroke of the piston becomes equal to or greater than a predetermined value. to provide an operating actuator of a friction fastening element.

[0026] According to this configuration, the zero-clearance position detecting means can detect the position of the piston as the actual zero-clearance position when the ratio of the change amount of the transmission torque of the friction fastening element to the change amount of the stroke of the piston becomes a predetermined value or more, and the accuracy in the case of torque control is higher than that in the case of stroke control of the electric motor. Thereby, for example, even when a deviation occurs between the actual release-side stroke region and the release-side stroke region at the time of design due to the temperature characteristics, manufacturing errors, wear, etc. of the friction plate, the actual zero-clearance position can be accurately detected.

Effect of the Invention

[0027] Provided is an operating actuator for a friction fastening element capable of achieving both an improvement in responsiveness during fastening of the friction fastening element and precise controllability of the fastening force.

Brief Description of the Drawings

[0028]

Figure 1

Figure 2

Figure 3

Figure 4

Figure 5

Figure 6

Figure 7

Figure 8

Figure 9

Embodiments for Carrying Out the Invention

[0029] Hereinafter, embodiments of the present invention will be described with reference to the accompanying drawings.

[0030] FIG. 1 is a schematic diagram of an operating actuator 2 of a friction fastening element 1 according to a first embodiment of the present invention, showing a released state of the operating actuator 2 of the friction fastening element 1. The friction fastening element 1 constitutes, for example, a plurality of clutches and brakes provided in an automatic transmission (not shown), and the operating actuator 2 is provided on each friction fastening element.

[0031] The clutch as the friction fastening element 1 includes a cylindrical drum 11, a cylindrical hub 12 having a smaller diameter than the drum 11, a plurality of friction plates 13 arranged axially between the drum 11 and the hub 12 and spline-engaged with the drum 11 and the hub 12 alternately, and an operating actuator 2 that presses the plurality of friction plates 13 to fasten them to each other. The plurality of friction plates 13 are pressed by a piston 23 provided in the operating actuator 2, whereby the clutch 1 is fastened.

[0032] Among the plurality of friction plates 13, the plurality of friction plates 13a arranged inside the drum 11 are spline-engaged with the drum 11 and arranged to be axially movable and rotatable integrally with the drum 11. The plurality of friction plates 13b arranged outside the hub 12 are spline-engaged with the hub 12 and arranged to be axially movable and rotatable integrally with the hub 12. The friction plates 13a and the friction plates 13b are arranged alternately and face each other via facings 13c provided on both sides of the friction plates 13b. On the other axial side of the plurality of friction plates 13, a retaining plate 15 that is spline-engaged with the drum 11 in the same manner as the friction plates 13a and is prevented from coming off by a snap ring 14 on one axial side is arranged.

[0033] The piston 23 is disposed on one axial side of the plurality of friction plates 13 and is fitted axially movably in a second cylinder 26b that forms part of an oil filling chamber 26 described later. On one axial side of the piston 23, a hydraulic chamber 27b that forms a second oil filling chamber described later is formed in an oil-tight state by the second cylinder 26b and the piston 23. On the other axial side of the piston 23, a return spring 16 that biases the piston 23 to one axial side (release side) is disposed.

[0034] The operating actuator 2 includes an electric motor 21, a screw mechanism 22 that converts the rotational force of the electric motor 21 into linear motion, a piston 23 that presses the friction plates 13, an oil-tight oil filling chamber 26 filled with hydraulic oil that transmits the linear motion of the screw mechanism 22 to the piston 23, and a hydraulic pressure generating piston 24 that is different from the piston 23 fitted in the oil filling chamber 26.

[0035] The electric motor 21 is constituted by, for example, a brushless DC motor including a rotor (not shown) made of a permanent magnet and a stator (not shown) made of a coil. A motor rotation speed sensor (for example, a magnetic sensor such as a Hall element) 35 is disposed on the stator side facing the magnetic poles of the rotor. The motor rotation speed is calculated from the period by synthesizing waveforms of a certain period based on the position detection signal from the Hall element. The position signal detected by the motor rotation speed sensor 35 is input to the control device 30 (see FIG. 4). The electric motor 21 is configured such that rotation speed control for controlling the rotation speed of the electric motor 21 and torque control for controlling the torque of the electric motor 21 can be switched, and the control of the electric motor 21 will be described later.

[0036] The screw mechanism 22 is, for example, a ball screw mechanism provided with a ball circulation mechanism in the form of a ball. The screw mechanism 22 includes a screw shaft 22a, a nut 22b that moves in the axial direction as the screw shaft 22a rotates, and a plurality of rolling elements (not shown) interposed between the screw shaft 22a and the nut 22b. The screw shaft 22a in the present embodiment is constituted by the rotating shaft of the electric motor 21, but it may be configured such that power is transmitted via a gear or the like to the rotating shaft of the electric motor 21.

[0037] On the outer peripheral surface of the screw shaft 22a, a substantially semi-circular arc-shaped ball rolling groove 22c is formed in a spiral shape. The ball rolling groove 22c is formed such that the pitch changes at different positions in the axial direction of the screw shaft 22a. The screw mechanism 22 in the present embodiment is constituted by a variable pitch ball screw mechanism.

[0038] On the inner peripheral surface of the nut 22b, a substantially semi-circular arc-shaped nut rolling groove (not shown) facing the ball rolling groove 22c of the screw shaft 22a is formed. The balls are arranged between the ball rolling groove 22c and the nut rolling groove (not shown). Thereby, the ball rolling groove 22c and the nut rolling groove are screwed together via the balls, and by rotating the screw shaft 22a by the electric motor 21, the balls circulate while rolling along the circulation path to move the nut 22b in the axial direction.

[0039] The oil filling chamber 26 includes a first oil filling chamber 27a formed in an oil-tight state by a first cylinder 26a filled with hydraulic oil and a hydraulic piston 24 fitted to the first cylinder 26a, a hydraulic chamber 27b as a second oil filling chamber formed in an oil-tight state by a second cylinder 26b and a piston 23 fitted to the second cylinder 26b, and a hydraulic pipeline 26c connecting the first oil filling chamber 27a and the second oil filling chamber 27b.

[0040] In the first oil filling chamber 27a, a reservoir tank 26e that can be communicatively connected to the first oil filling chamber 27a via a connection portion 26d provided in the first cylinder 26a is connected. The reservoir tank 26e is filled with hydraulic oil, and a check valve 26f for supplying hydraulic oil to the first oil filling chamber 27a is provided in the connection portion 26d, which is released only when the hydraulic pressure in the first oil filling chamber 27a becomes lower than the hydraulic pressure in the reservoir tank 26e.

[0041] The hydraulic pressure generating piston 24 is connected to the nut 22b via a connecting member 25, and is configured to move axially in conjunction with the axial movement of the nut 22b. Thus, when the axial movement of the nut 22b accompanying the rotation of the electric motor 21 of the screw mechanism 22 is transmitted to the hydraulic pressure generating piston 24 via the connecting member 25 and the hydraulic pressure generating piston 24 moves axially, the hydraulic oil in the first oil filling chamber 27a flows into the second oil filling chamber 27b via the hydraulic pipeline 26c, and the piston 23 strokes toward the fastening (the other axial direction) side.

[0042] In FIG. 1, the friction fastening element 1 is shown in a released state where the hydraulic pressure is discharged from the hydraulic chamber 27b and the piston 23 is moved toward the anti-friction plate 13 side by the biasing force of the return spring 16.

[0043] When the clutch 1 is fastened, if the electric motor 21 of the operating actuator 2 is rotated in the released state shown in FIG. 1 to stroke the nut 22b toward the fastening side, as shown in FIG. 2, the hydraulic pressure generating piston 24 moves axially to the other side in the first cylinder 26a. The axial movement of the hydraulic pressure generating piston 24 causes the hydraulic oil in the first cylinder 26a (the first oil filling chamber 27a) to flow into the second cylinder 26b (the second oil filling chamber 27b) via the hydraulic pipeline 26c, and the piston 23 moves toward the friction plate 13 side against the biasing force of the return spring 16.

[0044] At this time, the nut 22b strokes from the nut release position b1 on the screw shaft 22a corresponding to the predetermined release position a1 of the piston 23 to the nut zero clearance position b2 on the screw shaft 22a corresponding to the zero clearance position a2 of the piston 23. Due to the stroke of the nut 22b at this time, a holding hydraulic pressure P2 for holding the piston 23 at the zero clearance position a2 against the biasing force of the return spring 16 is supplied to the oil filling chamber 26 (hydraulic chamber 27b), which is lower than the fastening hydraulic pressure P1 corresponding to the pressing force required for fastening the friction fastening element 1 (see Fig. 5).

[0045] Furthermore, when the nut 22b is stroked toward the fastening side in the zero clearance state shown in Fig. 2, as shown in Fig. 3, the hydraulic piston 24 moves further axially toward the other side within the first cylinder 26a. Due to the axial movement of the hydraulic piston 24, the hydraulic oil in the first cylinder 26a flows into the second cylinder 26b (hydraulic chamber 27b) through the hydraulic pipeline 26c, increasing the hydraulic pressure in the hydraulic chamber 27b. The piston 23 presses the friction plate 13, and the friction plate 13 is sandwiched between the retaining plate 15 fixed to the drum 11 and the piston 23 to become non-rotatable relative to each other, thereby bringing the friction fastening element 1 into the fastened state.

[0046] At this time, the nut 22b strokes from the nut zero clearance position b2 to the nut fastening position b3 on the screw shaft 22a corresponding to the fastening position a3 where the piston 23 presses the friction plate 13 and the friction fastening element 1 is in the fastened state. Due to the stroke of the nut 22b at this time, a fastening hydraulic pressure P1 corresponding to the pressing force required for fastening the friction fastening element 1 is supplied to the oil filling chamber 26 (hydraulic chamber 27b) (see Fig. 5).

[0047] On the other hand, when releasing the friction fastening element 1, if the nut 22b is stroked toward the release side (from the nut fastening position b3 to the nut zero clearance position b2) in the fastened state shown in Fig. 3, as shown in Fig. 2, the pressing force by the piston 23 is released in a state where the piston 23 is in contact with or almost in contact with the friction plate 13, and the friction fastening element 1 becomes in the zero clearance state.

[0048] Further, when the nut 22b is stroked to the release side in the zero-clearance state shown in FIG. 2 (stroked from the nut zero-clearance position b2 to the nut release position b1), as shown in FIG. 1, the piston 23 moves toward the anti-friction plate 13 side by the biasing force of the return spring 16, and the friction engaging element 1 is released.

[0049] An automatic transmission (not shown) having a plurality of friction engaging elements 1 includes a control device 30 that controls each operating actuator 2 in each friction engaging element 1 to form a gear stage corresponding to the operating state. As shown in FIG. 4, signals from a range sensor 31 that detects a range selected by the driver's operation, signals from a vehicle speed sensor 32 that detects the vehicle speed of the vehicle on which the automatic transmission is mounted, signals from an accelerator opening sensor 33 that detects the operation amount (accelerator opening) of the driver's accelerator pedal, signals from an engine speed sensor 34 that detects the rotational speed of the engine mounted on the vehicle, a motor speed sensor 35 that detects the rotational speed of the electric motor 21 of the operating actuator 2, signals from a hydraulic pressure sensor 36 provided on the oil filling chamber 26, etc. are input to the control device 30. Note that the control device 30 is mainly configured by a microcomputer.

[0050] Based on various input signals, the control device 30 outputs control signals to each operating actuator 2 to control the automatic transmission. In the shift control of the automatic transmission, for example, the target gear stage is determined based on the vehicle speed detected by the vehicle speed sensor 32, the accelerator opening detected by the accelerator opening sensor 33, and a predetermined shift map.

[0051] When shifting to the target gear stage, the operating actuators 2 corresponding to the friction engaging element on the release side and the friction engaging element on the engagement side, for which so-called gear change is performed, are controlled.

[0052] When the friction engagement element 1 is engaged, in the release-side stroke region from the release position a1 of the piston 23 to the zero-clearance position a2, it is desirable to increase the stroke speed of the piston 23 due to the requirement of responsiveness, and it is necessary to increase only the stroke speed of the piston in the preset release-side stroke region by rotational speed control for controlling the rotational speed of the electric motor (hereinafter also referred to as "rotational speed").

[0053] On the other hand, in order to perform the engagement control of the friction engagement element well, it is necessary to precisely control the transmission torque of the friction engagement element while the piston moves in the engagement-side stroke region from the zero-clearance position a2 to the full-engagement position a3 where the friction engagement element 1 is fully engaged.

[0054] As shown in FIG. 5, the ratio of the change amount of the transmission torque of the friction engagement element 1 to the change amount of the stroke in the engagement-side stroke region is large, and the controllability is poor. Therefore, when the engagement control of the friction engagement element 1 is executed by controlling the stroke amount of the piston by controlling the rotational speed of the electric motor, precise engagement control is not executed in the engagement-side stroke region, and there is a risk of shock due to an unexpected change in the transmission torque.

[0055] The control device 30 is provided with a motor control unit 131 for controlling the electric motor 21, and the motor control unit 131 has a configuration that enables both improvement of responsiveness and precise controllability of the fastening force when the friction fastening element is fastened.

[0056] When performing shift control by the control device 30, the motor control unit 131 controls the rotational speed of the electric motor 21 so that the motor rotational speed becomes the target rotational speed to control the stroke amount of the piston 23 in the stroke control state, and based on the required torque from the driver (calculated from, for example, the accelerator opening), determines the target torque which is the control target value of the output torque of the electric motor 21 and selectively executes the torque control state for controlling the electric motor 21 so that the target torque is achieved.

[0057] When the electric motor 21 moves the piston 23 from a predetermined release position a1 to a zero clearance position a2 (when the piston 23 is located in a release side stroke region rather than the zero clearance position a2), stroke control is executed. When the piston 23 is moved from the zero clearance position a2 to a full fastening position a3 of the friction fastening element 1 (when the piston 23 is located in a fastening side stroke region rather than the zero clearance position a2), torque control is executed.

[0058] The motor control unit 131 controls a drive circuit (not shown) such as an inverter circuit for driving the electric motor 21 so that the electric motor 21 rotates at the maximum rotational speed in the stroke control state, and controls the drive circuit so that the electric motor 21 outputs a required output torque necessary for generating a required piston pressing force calculated from the required torque of the vehicle by the piston 23 in the torque control state.

[0059] The motor control unit 131 calculates the stroke amount and stroke position of the piston 23 for switching between the stroke control and torque control of the electric motor 21. The motor control unit 131 calculates the stroke amount of the piston 23 based on a signal from the motor rotation speed sensor 35. For example, based on a change in pulses corresponding to the rotation of the electric motor 21 output by the Hall element which is the motor rotation speed sensor 35, the rotation angle of the screw shaft 22a is calculated, and based on the calculated rotation angle of the screw shaft 22a, the stroke amount of the nut 22b determined from the pitch of the screw shaft 22a, the pressure receiving area of the hydraulic pressure generating piston 24, the pressure receiving area of the piston 23, etc., the stroke amount of the piston 23 is calculated. Note that the relationship between the rotation angle of the screw shaft 22a and the stroke amount of the piston 23 may be stored in advance as a map in the storage unit of the motor control unit 131, and the stroke amount of the piston 23 may be read from the map based on the calculated rotation angle of the screw shaft 22a.

[0060] The motor control unit 131 calculates the stroke position of the piston 23 based on the stroke amount of the piston 23. The stroke position of the piston 23 is calculated as the distance from a predetermined release position a1 of the piston 23 by using the predetermined release position a1 of the piston 23 as a reference position (zero) and adding the stroke amount of the piston 23 calculated at the predetermined release position a1.

[0061] The motor control unit 131 determines whether the calculated stroke position of the piston 23 is smaller than a zero clearance position a2 determined in advance at the time of design (whether the piston 23 is located on the release side with respect to the zero clearance position a2). When the stroke position of the piston 23 is located on the release side with respect to the zero clearance position a2 at the time of design, stroke control is executed. When the calculated stroke position of the piston 23 is equal to or greater than the zero clearance position a2 at the time of design (is located on the fastening side), torque control is executed.

[0062] In this way, in the motor control unit 131, the stroke control state and the torque control state of the electric motor 21 are switched based on whether the piston 23 is located in the release side stroke region or the fastening side stroke region. The stroke control state and the torque control state of the electric motor 21 may be switched based on the stroke amount of the piston 23. Specifically, when the stroke amount from the release position a1 of the piston 23 is smaller than the stroke amount from the predetermined release position a1 to the zero clearance position a2 determined in advance at the time of design, stroke control is executed. When the stroke amount from the release position a1 of the piston 23 is equal to or greater than the stroke amount from the predetermined release position a1 to the zero clearance position a2 determined in advance at the time of design, torque control is executed.

[0063] The motor control unit 131 calculates the required output torque required for the electric motor 21 to perform torque control of the electric motor 21. The motor control unit 131 calculates the required piston pressing force of the piston 23 from the required transmission torque obtained from the accelerator opening, the vehicle speed, and a prestored shift map, the pressure receiving area of the piston 23, the effective radius of the friction plate, and the friction coefficient of the friction plate. The motor control unit 131 calculates the required output torque of the electric motor 21 based on the calculated required piston pressing force, the pitch of the screw shaft 22a, the efficiency of the ball screw, etc.

[0064] FIG. 6 is a flowchart for explaining the control of the electric motor 21 when the friction engagement element 1 is engaged. As shown in FIG. 6, the control of the friction engagement element 1 in which the control state of the electric motor 21 is switched according to the position of the piston 23 (the stage at the time of engagement of the friction engagement element) is performed by the control device 30.

[0065] The control operation shown in FIG. 6 is started in a state where the target shift stage is determined based on the vehicle speed detected by the vehicle speed sensor 32, the accelerator opening detected by the accelerator opening sensor 33, and a predetermined shift map, and the piston 23 of the friction engagement element 1 that switches from release to engagement at the target shift stage is located at a predetermined release position a1. The fastening control of the friction engagement element 1 when the friction engagement element 1 moves from the released state to the engaged state will be described.

[0066] First, the control device 30 calculates the current stroke position of the piston 23. Specifically, the current stroke position is calculated from the predetermined release position a1 of the piston 23 and the stroke amount of the piston 23 in the previous control cycle (step S1). Since the fastening control of the piston 23 starts from the predetermined release position a1, the stroke amount of the piston 23 in the previous control cycle is 0.

[0067] In step S2, it is determined whether the current stroke position of the piston 23 calculated in step S1 is smaller than the zero clearance position a2. Specifically, it is determined whether the stroke position of the current piston 23 with respect to the release position a1, when the release position a1 is taken as the reference position (zero point), is smaller than the zero clearance position a2 of the piston 23 with respect to the predetermined release position a1.

[0068] The determination in step S2 may be made based on the stroke amount of the piston 23. In that case, it is determined whether the stroke amount of the piston 23 is smaller than the stroke amount from the predetermined release position a1 set at the time of design to the zero clearance position a2 at the time of design.

[0069] If the determination in step S2 is YES, that is, if the stroke position of the piston 23 is located in the release side stroke region with respect to the zero clearance position a2, the stroke control of the electric motor 21 is executed. In the stroke control, the rotational speed of the electric motor 21 is set to the maximum rotational speed, the piston 23 is stroked, and the flow is returned.

[0070] In step S1 of the returned control cycle, the current stroke position is calculated in the same manner. If the determination in step S2 is NO, that is, if the stroke position of the piston 23 is located in the fastening side stroke region with respect to the zero clearance position a2, the torque control of the electric motor 21 is executed.

[0071] In the torque control, the required piston pressing force of the piston 23 is calculated from the required torque of the vehicle in step S4. Specifically, the required piston pressing force of the piston 23 is calculated from the required torque of the vehicle obtained based on the accelerator opening, vehicle speed, and pre-stored shift map, the pressure receiving area of the piston 23, the effective radius of the friction plate, and the friction coefficient of the friction plate.

[0072] In the subsequent step S5, the required output torque of the electric motor 21 is calculated. Specifically, based on the required piston pressing force calculated in step S4, the pitch of the screw shaft 22a, the efficiency of the ball screw, etc., the required output torque required for the electric motor 21 to perform torque control of the electric motor 21 is calculated. In step S6, the output torque of the electric motor 21 is set to the required output torque calculated in step S5, and the piston 23 is stroked to the full fastening position a3 so that the flow returns. From the zero clearance position a2 to the full fastening position a3, for example, it may be controlled by the required output torque of the motor according to the change in the transmission torque required by slip control or the like.

[0073] In this way, in the release side stroke region where the ratio of the stroke change amount to the change amount of the transmission torque of the friction fastening element is larger than that in the fastening side stroke region, by stroke-controlling the electric motor 21, the responsiveness during fastening of the friction fastening element 1 can be improved.

[0074] Also, in the fastening side stroke region where the ratio of the change amount of the transmission torque of the friction fastening element 1 to the stroke change amount is larger than that in the release side stroke region, by torque-controlling the electric motor 21, precise control of the transmission torque during fastening of the friction fastening element 1 can be executed. Since the transmission torque of the friction fastening element 1 is proportional to the operating hydraulic pressure, the operating hydraulic pressure is proportional to the nut pressing force, and the nut pressing force is proportional to the motor torque, for example, based on the transmission torque, operating hydraulic pressure, and nut pressing force of the friction fastening element 1, by torque-controlling the electric motor, the transmission torque of the friction fastening element 1 can be controlled more directly than in the case of stroke control.

[0075] Furthermore, since the operating actuator 2 is composed of a ball screw mechanism 22, an oil-filled chamber 26 in an oil-tight state, and a piston 23, it is possible to suppress the energy loss for pressure holding in the non-fastening state, such as when driving an oil pump to suppress a decrease in hydraulic pressure due to oil leakage from a solenoid valve or the like for hydraulic pressure holding in the non-fastening state.

[0076] In the first embodiment, the configuration of switching the electric motor 21 from stroke control to torque control at the zero clearance position a2 during design was described. However, the ratio of the change amount of the transmission torque of the friction fastening element 1 to the change amount of the stroke of the piston 23 of the electric motor 21 may deviate from the zero clearance position a2 during design. In the second embodiment, the switching position of the control of the electric motor 21 is set with the stroke position of the piston 23 when the ratio (dσ / dx) of the change amount of the transmission torque to the change amount of the stroke becomes a predetermined value or more as the actual zero clearance position a4. The second embodiment will be described with reference to FIGS. 7 and 9. In the second embodiment, the method of switching the control state of the electric motor 21 is different from that of the first embodiment. For the configurations common to the first embodiment, the same reference numerals are given and the description thereof is omitted.

[0077] FIG. 7 shows the transmission torque of the friction fastening element 1 with respect to the stroke amount of the piston 23 in which a part of the fastening side stroke region in FIG. 5 is enlarged. As shown in FIG. 7, when the piston 23 is stroked to the fastening side from the zero clearance position a2 during design, for a while, the ratio (dσ1 / dx1) of the change amount of the transmission torque to the change amount of the stroke is small, and when the stroke is further advanced, the ratio (dσ2 / dx2) of the change amount of the transmission torque to the change amount of the stroke may become large. There is a region on the fastening side of the zero clearance position a2 during design where the stroke control has better controllability than the torque control. In the second embodiment, the position of the piston 23 where the ratio of the change amount of the transmission torque to the change amount of the stroke becomes a predetermined value or more and the controllability by torque control is improved compared to stroke control is set as the actual zero clearance position a4.

[0078] Here, the resolution of the electric motor 2 means the minimum value of the operation command of the electric motor 21. The rotational resolution dθm of the electric motor 21 indicates the minimum angle (the angle that engraves the rotation of the electric motor 21 with 1 pulse) when the electric motor 21 rotates. The torque resolution dTm of the electric motor 21 is defined as the minimum unit of the output torque output from the electric motor 21. For example, the rotational resolution of the electric motor 21 is set to 0.005 deg, and the torque resolution is set to 0.000064 Nm. Therefore, the control state of the electric motor 2 with high accuracy also differs depending on the relationship between the rotational angle resolution dθm and the torque resolution dTm of the electric motor 21 and the change amount of the transmission torque (dσ / dx) with respect to the change amount of the stroke.

[0079] Since the stroke amount is proportional to the rotational angle and the transmission torque is proportional to the motor output torque, the change amount of the transmission torque (dσ1 / dx1) with respect to the change amount of the stroke is replaced with the change amount dT of the motor torque and the change amount dθ of the motor rotational angle, and the detection method of the actual zero clearance position a4 will be described. The ratio (dTm / dθm) of the torque resolution to the rotational angle resolution of the electric motor 21 is set to a predetermined value, and when the ratio (dT / dθ) of the change amount of the transmission torque with respect to the change amount of the stroke becomes equal to or greater than the ratio (dTm / dθm) of the torque resolution to the rotational angle resolution of the electric motor 21 (dT / dθ≧dTm / dθm), the position of the piston 23 at that time is set as the actual zero clearance position a4.

[0080] When the ratio (dT / dθ) of the change amount of the transmission torque with respect to the change amount of the stroke is smaller than the ratio (dTm / dθm) of the torque resolution to the rotational angle resolution of the electric motor 21 (dT / dθ<dTm / dθm), the motor control unit 131 performs stroke control on the electric motor 21.

[0081] On the other hand, when the ratio (dT / dθ) of the change amount of the transmission torque with respect to the change amount of the stroke becomes equal to or greater than the ratio (dTm / dθm) of the torque resolution to the rotational angle resolution of the electric motor 21 (dT / dθ≧dTm / dθm), the motor control unit 131 performs torque control on the electric motor 21.

[0082] In this embodiment, the ratio (dσ / dx) of the change amount of the transmission torque with respect to the stroke amount uses the ratio (dT / dθ) of the change amount of the motor torque with respect to the change amount of the rotation angle of the electric motor 21.

[0083] FIG. 8 is a flowchart for explaining the control of the electric motor 21 when the friction fastening element 1 is fastened. As shown in FIG. 8, the control of the friction fastening element 1 in which the control state of the electric motor 21 is switched according to the position of the piston 23 (the stage at the time of fastening the friction fastening element) is performed by the control device 30.

[0084] The control operation shown in FIG. 8 is started in a state where the target shift stage is determined based on the vehicle speed detected by the vehicle speed sensor 32, the accelerator opening detected by the accelerator opening sensor 33, and a predetermined shift map, and the piston 23 of the friction fastening element 1 that makes a shift from release to fastening at the target shift stage is located at a predetermined release position a1. The fastening control of the friction fastening element 1 when the friction fastening element 1 moves from the released state to the fastened state will be described.

[0085] First, the control device 30 calculates the ratio dT / dθ of the change amount of the motor torque with respect to the motor rotation angle (step S1). Specifically, dT is the difference between the current motor torque T(n) and the motor torque T(n−1) in the previous control cycle, and dθ is the difference between the current motor rotation angle θ(n) and the motor rotation angle θ(n−1) in the previous control cycle. The change amount of the motor torque may be obtained by detecting the current value of the electric motor 21, or the rotation angle of the motor may be calculated based on a signal from the rotation speed sensor.

[0086] In step S2, it is determined whether the ratio dT / dθ of the change amount of the motor torque with respect to the motor rotation angle calculated in step S1 is smaller than the ratio (dTm / dθm) of the motor torque resolution with respect to the motor rotation angle resolution. If the determination in step S2 is YES, that is, if the ratio dT / dθ of the change amount of the motor torque with respect to the motor rotation angle is smaller than the ratio dTm / dθm of the motor torque resolution with respect to the motor rotation angle resolution, the stroke control of the electric motor 21 is executed (step S3). In the stroke control, the rotation speed of the electric motor 21 is set to the maximum rotation speed, the piston 23 is stroked, and the flow is returned.

[0087] In step S1 of the returned control cycle, the ratio dT / dθ of the change amount of the motor torque with respect to the motor rotation angle is similarly calculated. If the determination in step S2 is NO, that is, if the ratio dT / dθ of the change amount of the motor torque with respect to the motor rotation angle is greater than or equal to the ratio dTm / dθm of the motor torque resolution with respect to the motor rotation angle resolution, the torque control of the electric motor 21 is executed.

[0088] In the torque control, the required piston pressing force of the piston 23 is calculated from the required torque of the vehicle in step S4. Specifically, the required piston pressing force of the piston 23 is calculated from the required torque of the vehicle obtained by the accelerator opening degree, the vehicle speed, and a prestored shift map, the pressure receiving area of the piston 23, the effective radius of the friction plate, and the friction coefficient of the friction plate.

[0089] In the subsequent step S5, the required output torque of the electric motor 21 is calculated. Specifically, based on the required piston pressing force calculated in step S4, the pitch of the screw shaft 22a, the efficiency of the ball screw, etc., the required output torque required for the electric motor 21 to perform torque control of the electric motor 21 is calculated. In step S6, the output torque of the electric motor 21 is set to the required output torque calculated in step S5, and the piston 23 is stroked to the full fastening position a3 so that the flow is returned. From the zero clearance position a2 to the full fastening position a3, for example, it may be controlled by the required output torque of the motor according to the change in the transmission torque required by slip control or the like.

[0090] According to the second embodiment, when the ratio of the change amount of the transmission torque of the friction fastening element to the change amount of the stroke of the piston 23 is equal to or greater than a predetermined value, the accuracy in the case of torque control is higher than that in the case of stroke control of the electric motor 21. The position of the piston 23 at this time can be detected as the actual zero clearance position a4. Thereby, for example, even when a deviation occurs between the actual release side stroke region and the release side stroke region at the time of design due to the temperature characteristics of the friction plate, manufacturing errors, wear, etc., the actual zero clearance position can be accurately detected.

[0091] The zero clearance position may be detected using a hydraulic pressure sensor 36 as piston position detection means provided in the oil filling chamber 26 (see FIG. 1). The detection of the actual zero clearance position a4 is set such that the value of the hydraulic pressure sensor 36 becomes a hydraulic pressure equal to or higher than a predetermined threshold value P3, as shown in FIG. 5. The predetermined threshold value P3 is set to a hydraulic pressure equal to or higher than the holding hydraulic pressure P2 that holds the state where the clearances between the plurality of friction plates 13 are filled against the biasing force of the return spring 16 by the piston 23, for example.

[0092] Further, the piston position detection means may be constituted by a hydraulic pressure switch. Thereby, the zero clearance position can be detected with a less expensive configuration compared to the case of using a hydraulic pressure sensor or the like. Further, the piston position detection means may be constituted by a torque sensor that detects the rise of torque.

[0093] In addition, the operating actuator 2 of the friction fastening element 1 is provided with a screw mechanism 22 for achieving both an improvement in responsiveness during fastening and ensuring a pressing force while miniaturizing the motor. As shown in FIG. 9, the screw mechanism 22 is constituted by a variable pitch ball screw mechanism, and for example, a screw mechanism disclosed in Japanese Patent No. 4366215 or the like can be used. The screw mechanism 22 includes a first pitch region X1 in which a first pitch X11 is formed on one side in the axial direction of the screw shaft 22a, and a second pitch region X2 that is continuous with the first pitch region X1 and in which a second pitch X21 is formed on the other side in the axial direction. The second pitch X21 is formed to be smaller than the first pitch X11. In the present embodiment, as disclosed in Japanese Patent No. 4366215, the nut 22b is constituted by a ball holding member (not shown) having a rolling element transfer groove and a case (not shown) in which the ball holding member is rotatably accommodated, and is configured such that the ball holding member rotates with respect to the case in correspondence with a change in the pitch of the screw shaft.

[0094] The pitches X11 and X21 provided on the screw shaft 22a are the distances between adjacent screw grooves. In the present embodiment, since the screw shaft 22a is constituted by a single-thread screw, one pitch coincides with the distance that the nut 22b moves in the axial direction when the screw shaft 22a (electric motor 21) rotates once. The stroke amount of the nut when the electric motor 21 rotates once is such that the stroke amount of the first pitch region X1 is larger than the stroke amount of the second pitch region X2. Therefore, the stroke speed of the nut 22b in the first pitch region X1 is set to be faster than the stroke speed of the nut 22b in the second pitch region X2.

[0095] As described above, the piston 23 strokes by the stroke of the nut 22b. With reference to FIG. 9, the relationship between the first pitch region X1 and the second pitch region X2 where the stroke speed of the nut 22b changes and the stroke speed of the piston 23 will be described. Here, for the sake of easy understanding, for example, the stroke amount of the nut 22b and the stroke amount of the piston 23 are considered to be the same.

[0096] The first pitch region X1 is set to be the same as the stroke amount L1 from the preset release position a1 of the piston 23 to the zero clearance position (the zero clearance position at the time of design in the first and second embodiments) a2. The second pitch region X2 extends continuously to the other axial side from the first pitch region X1 and is set to be equal to or greater than the stroke amount from the preset zero clearance position a2 to the fastening position a3.

[0097] The nut release position b1 corresponding to the case where the piston 23 is located at the predetermined release position a1 is set such that the tip position of the nut 22b is located at one axial end of the first pitch region X1. The nut zero clearance position b2 corresponding to the case where the piston 23 is located at the zero clearance position a2 at the time of design is set such that the tip position of the nut 22b is located at the boundary position (one axial end of the first pitch region X1 and one axial end of the second pitch region X2) X of the first pitch region X1 and the second pitch region X2 as shown by the broken line. In other words, at the zero clearance position a2 at the time of design of the piston 23, the first pitch X11 and the second pitch X21 are set to be switched.

[0098] As is well known, since the motor torque and the motor rotation speed have an inverse proportional relationship, when the output of the electric motor is constant, if the motor torque is increased, the rotation speed decreases, and if the rotation speed is increased, the torque decreases.

[0099] Since the first pitch region X1 corresponds to the stroke amount L1 from the release position a1 of the piston 23 to the zero clearance position a2 at the time of design, the torque required for the electric motor 21 corresponding to the holding hydraulic pressure P2 is lower than the fastening hydraulic pressure P1, and the nut 22b can be moved at a high rotational speed without reducing the motor rotational speed and at a stroke speed faster than that of the second pitch region X2.

[0100] On the other hand, since the second pitch region X2 corresponds to the stroke amount L2 from the zero clearance position a2 at the time of design of the piston 23 to the fastening position a3, a large torque corresponding to the fastening hydraulic pressure P1 is required for the electric motor 21. Along with this, the motor rotational speed decreases and the nut 22b can be moved at a stroke speed slower than that of the first pitch region X1.

[0101] Therefore, when fastening the friction fastening element 1, if the nut 22b is stroked on the first pitch region X1, the piston 23 will be in a zero clearance state. If the nut 22b is further stroked on the second pitch region X2 in the zero clearance state, the friction plate 13 will be pressed substantially simultaneously with the stroke of the nut 22b, and the friction fastening element 1 will be fastened with good responsiveness.

[0102] Although the configuration in which the screw shaft 22a has the first pitch X11 and the second pitch X21 has been described, the pitch provided on the screw shaft 22a may be composed of three or more types of pitches. For example, a stepped region having a stepped pitch formed with a pitch smaller than the first pitch X11 and larger than the second pitch X21 may be formed at the boundary portion connecting the first pitch region X1 and the second pitch region X2. In the stepped region, the pitch may be set to gradually decrease from the first pitch region X1 toward the second pitch region X2. In this case, the zero clearance position a2 at the time of design may be set in the stepped region.

[0103] In this embodiment, a configuration of the oil filling chamber 26 including a first cylinder 26a, a second cylinder 26b, and a hydraulic pipeline 26c fluidly connecting the first cylinder 26a and the second cylinder 26b has been described. However, the oil filling chamber 26 may be configured to include either the first cylinder 26a or the second cylinder 26b.

[0104] In this embodiment, the electric motor 21 of the operating actuator 2 is described by taking the example that it is a brushless motor. However, the present invention is not limited to this, and other motors such as a stepping motor may be used.

[0105] In this embodiment, a configuration using a ball screw mechanism for the screw mechanism has been described. However, the present invention is not limited to this, and other feed screw mechanisms may be used.

[0106] In this embodiment, an example where the friction fastening element 1 is a clutch is shown. However, for a brake that connects a non-rotating element such as a transmission case and a hub 12 as a rotating member by a drum 11, basically, a similar operating actuator 2 may be provided.

[0107] The present invention is not limited to the illustrated embodiments, and various improvements and design changes are possible without departing from the gist of the present invention.

Industrial Applicability

[0108] As described above, according to the present invention, in the operating actuator of the friction fastening element, it is possible to achieve both an improvement in responsiveness during fastening of the friction fastening element and precise controllability of the fastening force. Therefore, it may be suitably used in the manufacturing industry field of the friction fastening element.

Explanation of Reference Numerals

[0109] 1 Friction fastening element 2 Operating actuator 11 Drum 12 Hub 13 Plurality of friction plates 21 Electric motor 22 Screw mechanism 22a Screw shaft 22b Nut 23 Piston 26 Oil filling chamber 36 Zero clearance position detecting means (hydraulic sensor, hydraulic switch) a1 Predetermined release position a2 Zero clearance position X1 First pitch region X2 Second pitch region X11 First pitch X21 Second pitch

Claims

Claim 1. An operating actuator of a friction fastening element that is provided between a drum and a hub and presses a plurality of friction plates that alternately engage with the drum and the hub to fasten them to each other, comprising an electric motor, a screw mechanism that converts the rotational force of the electric motor into linear motion, a piston that presses the friction plates, and an oil-filled chamber in an oil-tight state filled with hydraulic oil that transmits the linear motion of the screw mechanism to the piston, wherein the screw mechanism has a screw shaft and a nut that moves axially on the screw shaft and transmits a pressing force to the piston via the oil-filled chamber, wherein the electric motor, stroke control is executed to control the stroke amount of the piston by controlling the rotational speed of the electric motor when moving the piston from a predetermined release position to a zero-clearance position where the clearance between the friction plates is filled and the plurality of friction plates are in a zero-clearance state, and torque control is executed to control the transmission torque of the friction fastening element by controlling the output torque of the electric motor when moving the piston from the zero-clearance position to a predetermined fastening position, wherein the screw mechanism is constituted by a ball screw mechanism, wherein the screw shaft includes a first pitch region in which a first pitch is formed on one side in the axial direction and a second pitch region in which a second pitch is formed on the other side in the axial direction continuously with the first pitch region, and the second pitch is formed to be smaller than the first pitch, wherein the first pitch and the second pitch are set to switch at the zero-clearance position, an operating actuator of a friction fastening element.

2. further comprising zero-clearance position detection means for detecting the zero-clearance position, wherein the zero-clearance position detection means is configured to detect the zero-clearance position of the piston at the rise of the hydraulic pressure in the oil-filled chamber, the operating actuator of the friction fastening element according to claim 1.

3. wherein the zero-clearance position detection means is constituted by a hydraulic switch, the operating actuator of the friction fastening element according to claim 2.

4. wherein the zero-clearance position detection means is constituted by a hydraulic sensor, the operating actuator of the friction fastening element according to claim 2. Claim 5. An operating actuator of a friction fastening element that is provided between a drum and a hub and presses a plurality of friction plates that alternately engage with the drum and the hub to fasten them to each other, An electric motor, a screw mechanism that converts the rotational force of the electric motor into linear motion, a piston that presses the friction plate, and an oil-filled chamber in an oil-tight state filled with hydraulic oil that transmits the linear motion of the screw mechanism to the piston. The screw mechanism includes a screw shaft and a nut that moves axially on the screw shaft and transmits a pressing force to the piston via the oil-filled chamber. The electric motor Stroke control is executed to control the stroke amount of the piston by controlling the rotational speed of the electric motor when moving the piston from a predetermined release position to a zero-clearance position where the clearance between the friction plates is filled and the plurality of friction plates are in a zero-clearance state. Torque control is executed to control the transmission torque of the friction fastening element by controlling the output torque of the electric motor when moving the piston from the zero-clearance position to a predetermined fastening position. It further includes zero-clearance position detection means for detecting the zero-clearance position. The zero-clearance position detection means is an operating actuator of the friction fastening element that detects the zero-clearance position of the piston when the ratio of the change amount of the transmission torque of the friction fastening element to the change amount of the stroke of the piston becomes equal to or greater than a predetermined value.

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